Optical microprobe devices and methods are utilized in various photonic and biomedical applications where it is desirable to locally deliver the optical power from lasers, other radiation sources, and the like to small areas of modified or examined samples. For example, it may be desirable to cut or otherwise modify biological tissue or other material using light; collect reflected, scattered, or emitted light from a sample; encode data to/decode data from a material using light; etc. In such applications, the spatial resolution of the optical microprobe devices and methods is governed by the diameter of the focused light beam in a medium or sample under study—a key parameter. Typically, the spatial resolution of optical devices operating in far field is determined by the wavelength of the light utilized and the aperture of the objective-lens system, i.e. the diffraction limit. The efficient delivery of optical power is another key parameter, with greater optical power typically being desirable. The task of development of compact light-focusing tools for photonics and biomedical applications is challenging due to the multimodal structure of the beams propagating in the often flexible delivery systems of such devices. The potential applications of such technologies are endless, including nanoscale patterning, the formation of tiny holes in thin films, ultra-precise laser tissue surgery, the piercing of a cell, the spectroscopic characterization of individual particles, and the like.
Conventional optical microprobe devices and methods suffer from a number of significant shortcomings. Existing near-field optical microprobes have high resolution, but extremely limited optical transmission. Utilizing tapered optical fibers coated with opaque metallic films and tiny transmissive apertures, smaller spot sizes are obtainable, i.e. less than λ (10-20 nm, for example). However, these optical microprobes typically fail to deliver adequate optical power. Existing far-field optical microprobes have high optical transmission properties, but very limited spatial resolution, especially in the case of multimodal input beams. Utilizing solid immersion lenses (SILs) or the like and perfectly collimated or conical beams of light, smaller spot sizes are also obtainable, i.e. less than λ (λ/2, for example). However, such perfectly conical beams of light are not readily available in conventional optical delivery systems used in laser-tissue surgery and other applications. Typically, obtaining such perfectly conical beams of light requires the use of single mode optical fibers as the means of optical delivery. It should be noted, however, that single mode optical fibers are not readily available in the mid-infrared range of the spectrum. They also have limited coupling efficiency with many practical radiation sources, and limited power transmission properties. Instead, using multimodal beams in such systems results in greatly diminished spatial resolution, well below the diffraction limit. In any event, such optical microprobes, and others, do not allow for direct contact with a sample, i.e. tissue contact in a biomedical application, due to high refractive indices (˜1.33 or larger) of the later, leading to defocusing of the beam. It should be noted, however, that such a contact mode of operation may be desirable in some applications, such as ultra-precise laser-tissue surgery applications, etc. Other existing optical microprobes may obtain better spatial resolution than the diffraction limit in an imaging mode, but require special spectral characteristics and material properties, such as fluorescence, nonlinearity, etc. These optical microprobes also typically fail to deliver adequate optical power. Further existing optical microprobes utilize exotic metamaterials with a negative index of refraction, permittivity, and permeability and are extremely challenging to fabricate. In addition, these exotic metamaterials have a limited usable frequency range. Still further existing optical microprobes utilize single transparent dielectric microspheres or microcylinders (with wavelength-scale dimensions) and generate tightly focused beams referred to as photonic nanojets that are comparable to the diffraction limit laterally and one or two wavelengths long with reasonably small losses, but these photonic nanojets may be obtained only in the case of perfect plane wave illumination. In addition, such optical microprobes require the use of single mode fibers (SMFs) or inflexible waveguides, not readily available in or suitable for many applications. A concrete example of such an application is laser-tissue surgery, which is typically performed in the mid-IR range, where SMF is currently not available. Most practical radiation sources available provide diverging multimode beams of light. In most practical applications, the use of robust and flexible waveguides, including optical fibers (i.e. multi mode fiber (MMF)) and hollow waveguides is desirable. Finally, in all of the above cases, it is difficult to control the separation between the tip of the microprobe and the sample, also referred to as the “working distance,” with the accuracy required. More importantly, if the focusing of light is required in a contact mode, it is practically impossible to achieve wavelength-scale focused spot sizes in sample/tissue for microprobes operating in a far field due to the high refractive index of the sample/tissue.
Thus, what are still needed in the art are optical microprobe devices and methods that provide reasonably high spatial resolution (on the order of λ or smaller), the efficient transmission of optical power from multimodal radiation source to sample, have a readily controllable working distance with a simple setup, are capable of being operated in a contact mode, and that incorporate robust and flexible optical fiber or hollow waveguides, all without the need for “perfect” radiation sources or exotic, expensive materials.